Robotic media storage libraries, or data storage libraries, are devices for providing automated access to a large collection of data stored on multiple physical storage media, such as magnetic tape cartridges, disks, compact discs, or DVDs, for example. Data storage libraries generally contain a plurality of storage locations or slots for storing individual storage elements, one or more media drives for reading or writing physical media, and an access device or robotic “picker” for moving storage elements from a storage location to a drive and back. Robotic media storage libraries may have special storage locations designated for certain purposes, such as serving as a temporary storage location while two pieces of media are being swapped or for adding or removing physical media from the library.
Library storage density is limited to the actual volume of the device used to store the physical data cartridges or disks. In some cases this means limited floor space or area, or limited vertical shelf or wall space, depending on the particular configuration of the storage library.
In the typical library scheme, storage capacity is increased by increasing the size of the library itself. This requires increased floor space or increased wall space, depending on the type of library used. The area accessible to a retrieval device also limits the storage capacity of a library, because a retrieval device must have a path to reach each individual data storage element and, in most cases, the drives. Additionally, operator access to areas within large libraries for maintenance purposes also requires a certain amount of added room. These goals of decreased overall volume and increased accessible area conflict in storage library design, making storage density a key factor in the library marketplace.
There are several existing schemes for storage libraries. For example, libraries using concentric “silo” storage arrays, parallel linear wall arrays, and U-shaped arrays are known in the art. Of these examples, U-shaped typically provide the best storage density. Some previous designs are considered briefly below.
FIG. 1 shows an example of a small, self-contained data storage library of a previous design. The library comprises an area surrounded by walls that enclose individual data storage elements 102, robotic access device 104, and tape drives 106 which read individual data storage elements 102. In this example, there are two parallel rows of storage slots for individual data storage elements. These rows of storage slots can be two dimensional arrays of storage slots, or stacked one dimensional arrays of storage slots. Between the two storage arrays is track 108 for access device 104, typically a robotic picker of some kind that can remove and replace data storage elements 102 from their slots and transport them to drives 106. Access device 104 is designed to have access to any slots in either array. This sample configuration also shows the storage sots extend beyond the parallel sections of the library to curve around toward the drives, adding capacity to the library.
FIG. 2A shows a large storage library of a previous design. In this example, storage arrays 202, 204, 206, 208 (which hold individual storage elements) are positioned similar to the example of FIG. 1 except that additional arrays 202, 208 are positioned on the “backside” of each array 204 and 206, forming four separate arrays or walls of storage slots. This figure shows two robotic access devices 210 which translate along track 212 to access individual storage elements. Also shown are drives 214 for reading the data storage elements. Robotic access devices 210 in this example remove the storage elements from a slot and translate the element to drives 214, then back again as necessary. As the figure shows, there is a large amount of unused space in this design between the two sets of storage arrays. The positions of the arrays, i.e., two sets back to back on either side of the library, requires room on both sides of the arrays for the robot to access the storage elements therein.
FIG. 2B shows another kind of known storage library. In this example, the library comprises inner 216 and outer 218 rings of storage slots. The individual storage elements in each slot are accessed by arm 210 that rotates on an axis concentric with the centers of the rings. Media drives 214 are positioned to receive retrieved storage elements from the arm. In this configuration, the robot arm must be able to reach through the inner ring to the outer ring, requiring the use of valuable space that could otherwise be used for storage elements.
FIG. 2C shows a top view of yet another storage system of previous design. This example depicts a simple, straight forward linear array of storage slots which are accessed by a robotic picker of some kind 210. Robot 210 moves the individual storage elements from their storage slot to media drives 214 and back again. Access ports 220, which can serve as staging areas for shuffling storage elements, are also shown.
In both small and large libraries, limited space provides a constant impetus to increase storage density, and it is to this problem that the present innovation is directed.